The action of fibrinolysin on ovine prolactin has been investigated. It was found that the predominant action of fibrinolysin on ovine prolactin is the cleavage of the Met-Ala bond at positions 53-54. Thus, the main product of a limited fibrinolysin digestion of prolactin are PRL-(1-53) and PRL-(54-199). Both fragments are highly soluble in aqueous solutions of low ionic strength. From CD spectra they retain some alpha-helix and their side-chain spectra differ considerably from that of the parent molecule. Immunodiffusion and complement fixation data show that both fragments are not immunoreactive with the antiserum to the parent molecule. Bioassay of the two fragments in the pigeon crop-sac test also indicates that they do not possess any lactogenic activity (Table 2). Apparently the cleavage of a single peptide bond in the prolactin molecule into two fragments destroys the antigenic recognition sites for the prolactin antibody as well as the biological activity. Experimental data provides evidence that two peptide fragments, PRL-(1-53) and PRL-(54-199), from ovine prolactin are capable of recombination by non-covalent interaction to regenerate full immunoreactivity but little hormonal activity (see Table 2). Exclusion chromatography and circular dichroism of the recombinant appear to behave as the natural hormone. In the case of HGH, the recombinant exhibits full growth-promoting activity as well as immunoreactivity. It is apparent that the tertiary structure of the prolactin recombinant is not completely restored to its original conformation which is required for biological activity, but not required for immunoreactivity.